[Eric]

This year i encountered more tough grammar questions than all my previous years...

i usually use your first method, run through a bunch of examples until a distinction or rhyme or reason shows itself. If that doesn't work or the students demand more, i tell them to see me after class or i take a look at a grammar book or ask another teacher during a break.

i think that specific example you gave definitely justifies the "Stop splitting hairs" response you gave them. essentially, there is no difference, but the tiny differences that you found were quite good.